In these environmentally aware times, there is an increasingly growing demand for hybrid vehicles which provide improved fuel economy and reduced exhaust gas emissions. Most of the hybrid vehicles now commercially available employ a drive scheme (of a so-called split type) described Japanese Patent Laid-Open Publication No. Hei 11-22501. Under low-speed and low-load conditions of the vehicle in which the fuel economy of the internal combustion engine is degraded, this drive scheme is adapted to improve fuel economy by driving the vehicle only by the motor with the internal combustion engine maintained at rest. On the other hand, when the high-voltage battery is in a low state of charge or under high-load conditions, this scheme is also adapted to start the internal combustion engine to drive the vehicle using the power from the engine. To launch and accelerate the hybrid vehicle from rest, the scheme is further adapted to accelerate the vehicle using the power from the motor, while starting the internal combustion engine using a generator during the launch and acceleration.
When the internal combustion engine is started using the generator, power is supplied from the high-voltage battery to the generator for motoring the internal combustion engine or part of split generator torque is transmitted to the driveshaft of the traction wheels as the vehicle is started. Thus, with no countermeasures taken against this, changes would occur in vehicle drive torque causing the driver to feel torque shocks. To address this problem, a system has been developed which cancels the generator torque transmitted to the driveshaft of the traction wheels with the motor torque in order to prevent changes in vehicle drive torque. When the internal combustion engine is started using the generator but not smoothly, such a system will experience motoring by the generator or increase in time for torque canceling by the motor, resulting in an increase in power consumption in the high-voltage battery. On the other hand, during the startup, the running drive power in the motor drive mode is sourced only from the high-voltage battery energy, thereby causing a further increase in power consumption in the high-voltage battery. This in turn may make it necessary to limit the running drive power in the motor drive mode depending on the capacity of the high-voltage battery, causing the driver to feel lack in power. Accordingly, it is a critical technical issue to provide an improved startup capability for the internal combustion engine of a split-type hybrid vehicle.
One of the conventional technologies for improving the startup capability of an internal combustion engine is disclosed in Japanese Patent Laid-Open Publication No. 2000-213375, entitled “Control method and apparatus for stopping internal combustion engines.” The technique disclosed in this patent document is intended to improve the startup capability such that when the internal combustion engine stops, the crankshaft of the internal combustion engine is stopped at a predetermined crank position suitable for startup.
Applying the technique disclosed in this patent document mentioned above to the split-type hybrid vehicle may raise the following problems.
That is, the crankshaft may be stopped at a predetermined crank position when the internal combustion engine stops. However, when only the motor is used to drive the vehicle, a planetary gear mechanism coupling between the motor and the generator would cause the planetary gear of the planetary gear mechanism to rotate (on its axis) depending on the running speed, thereby causing the generator to rotate. Accordingly, a change in vehicle speed would correspondingly change the speed of the generator, thereby causing a reactive force due to generator inertia to act upon the crankshaft of the internal combustion engine. This reactive force may change the crank position of the internal combustion engine to dislocate the crank position from a suitable startup crank position. This may result in degradation in startup capability, thereby causing an increase in power consumption in the high-voltage battery. As described above, it will be thus necessary to limit the running drive power provided only by the motor, thereby causing the driver to feel lack in power. From the viewpoint of reduction in battery costs and vehicle weight, this problem will become more noticeable as the high-voltage battery mounted in the vehicle is reduced in capacity.